The present invention relates, in general, to temperature control circuits for integrated circuits, and more particularly, to temperature control circuits for power transistors.
Power transistors, including bipolar and MOSFET transistors, are well known. Power transistors are used to control high currents delivered to a load. Both bipolar and MOSFET power transistors have control electrodes, base or gate, which control current flowing through the transistor and allow it to be switched on and off. Power transistors are different from typical integrated circuits in that they operate at high temperature, typically 100 to 175.degree. C., and must dissipate a large amount of heat through the package in which they are encapsulated.
Although the processes involved in making a power transistor and making typical integrated circuits are similar, they are not usually compatible. Attempts have been made to integrate control functions with a power transistor, but these attempts have not met with commercial success. This is because the additional cost involved in meshing both a standard integrated circuit process and a power transistor process make the resulting combination cost more than either of the devices alone. Also, combining two disparate processes compromises performance of both the integrated circuit and the power transistor. Because of this, many users have found it necessary to add control circuitry external to the power transistor to perform functions such as current limiting and temperature limiting.
Integrated circuit performance is usually limited to a specific operating temperature range. Power transistors in particular, because they already operate at high temperature, are particularly sensitive to an upper operating temperature limit. When a power transistor is forced to perform above its operating temperature limit, even in a transient mode, the device is often destroyed. Because of this, it has long been desired to be able to shut off a power transistor when a predetermined temperature or current limit has been reached to protect the device from destruction. As set out hereinbefore, however, it has been difficult to integrate such protection circuitry onto the same chip as the power transistor.
Effectiveness of a protective circuit is largely a function of how quickly it responds to a condition which has endangered the power device. A protection circuit must respond quickly, while at the same time not acting so quickly that spurious turn off results. Thus, just making the protective circuit more sensitive is not usually an acceptable means of improving response time because this will lead to false triggering of the protection circuit. What is needed is a method of quickly generating a signal which is proportional to the magnitude of the condition which endangers the power device.
Generating a temperature protection signal in particular has been difficult. Two aspects of thermal protection are important: first, the protection circuitry must be heated before response occurs, and second, the protection circuitry must be coupled to the power transistor by a low thermal impedance coupling. To speed response time, the thermal mass of the protection device must be minimized and the protection device must be located very close to the source of heat in the power transistor. If the protection circuitry is located in a different package from the power device, the only thermal coupling between the power device and the protection circuitry would be if the two devices shared a common heatsink. This thermal coupling is very slow and is effective only in very crude circuits. Another method is to mount the protection circuitry and the power transistor side-by-side in the same package which greatly reduces the physical space between the two circuits. This method, however, shares the same difficulty as the common heatsink method in that the time required for the thermal information to transfer from the power chip to the protection circuitry is longer than the time required to destroy the power transistor. So while these methods are effective for steady-state temperature protection, they are ineffective for transient thermal protection.
Surprisingly, even when the protection circuitry is formed on the same chip with the power transistor, the time delay between an over temperature condition and the protection circuitry responding to the over temperature condition can be too long. This is because the thermal mass of the entire power/protection chip must be heated to generate a response signal. Often, portions of the power device are heated to destructive temperatures before the entire mass is hot enough to trigger the protection circuitry. Until now, it has been very difficult to effectively protect a power transistor from transient thermal conditions.
Another method of coupling a protection circuit to a power transistor has been to mount the protection circuitry on top of the power transistor using a conductive paste to attach the back side of a protection circuit to the front side of a power transistor. This method further reduces the physical space between the protection circuitry and the power transistor, and allows the protection circuitry to approach the power transistor temperature relatively quickly. However, this method still suffers from the fact that heat transfer in silicon is relatively slow and that the entire mass of the power transistor and the protection circuitry had to rise in temperature before a shut off signal could be generated. Thus, even when the protection circuitry was mounted directly on the power transistor in this fashion the time delay between the transient and thermal condition and response of the protection circuitry was too long to protect the power transistor.
Accordingly, it is an object of the present invention to provide a power transistor with effective thermal protection.
Another object of the present invention is to provide a thermally protected power transistor with a faster response time to a thermal condition.
Another object of the present invention is to provide a thermally protected power transistor which is cost effective.
Another object of the present invention is to provide a thermally protected power transistor using conventional semiconductor processing techniques.
A further object of the present invention is to provide a thermally protected power transistor with improved response to transient thermal conditions.